Cathodic protection systems for supplying current to an anode to polarize a submersible metal unit such as a marine drive unit are well known. One such system is disclosed in U.S. Pat. No. 4,322,633 to the present inventor. In that system a control system controls the current supplied to the anode in response to the potential sensed by a reference electrode. Though that system has been highly effective, it requires the reference electrode to be mounted a substantial distance from the anode to provide an appropriate signal indicative of the potential of the protected unit, particularly when used in both fresh and salt water.
Another system, described in U.S. patent application Ser. No. 402,191, filed on July 26, 1982 by the present inventor, now U.S. Pat. No. 4,492,877, discloses an electrode apparatus for a cathodic protection system which uses a grounded shield mounted between the anode and the reference electrode to allow the anode and reference electrode to be mounted in close proximity to each other.